詳解Mysql case then應用

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表的創立

CREATE TABLE `lee` (
`id` int(10) NOT NULL AUTO_INCREMENT, 
`name` char(20) DEFAULT NULL, 
`birthday` datetime DEFAULT NULL, 
PRIMARY KEY (`id`)) ENGINE=InnoDB DEFAULT CHARSET=utf8

數據拔出：

insert into lee(name,birthday) values ('sam','1990-01-01');
insert into lee(name,birthday) values ('lee','1980-01-01');
insert into lee(name,birthday) values ('john','1985-01-01');

第一種用法:

SELECT name,
 CASE WHEN birthday < '1981' THEN 'old' 
WHEN birthday > '1988' THEN 'yong'
 ELSE 'ok' END YORN
FROM lee

第二種用法：

SELECT NAME, CASE name
 WHEN 'sam' THEN 'yong'
 WHEN 'lee' THEN 'handsome'
 ELSE 'good' END as oldname
FROM lee

第三種：固然了，case when 語句還可以復合

select name, birthday,
 case 
when birthday > '1983' then 'yong'
 when name='lee' then 'handsome'
 else 'just so so' end
from lee;

在這裡用sql語句停止日期比擬的話，須要對年加引號，要否則能夠成果和預期的成果分歧，
固然也能夠用year函數來完成

select name,
 case when year(birthday) > 1988 then 'yong'
 when year(birthday) < 1980 then 'old'
 else 'ok' END
from lee;
==========================================================
create table penalties
(
 paymentno INTEGER not NULL,
 payment_date DATE not null,
 amount DECIMAL(7,2) not null,
 primary key(paymentno)
)
insert into penalties values(1,'2008-01-01',3.45);
insert into penalties values(2,'2009-01-01',50.45);
insert into penalties values(3,'2008-07-01',80.45);

第一題：對罰款掛號分為三類，第一類low,包含年夜於0小於等於40的罰款，第二類moderate年夜於40到80之間的罰款，第三類high包括一切年夜於80的罰款

select payment_date, amount,
 case 
when amount >= 0 AND amount < 40 then 'low'
 when amount >=40 AND amount < 80 then 'moderate'
 when amount >=80 then 'high' 
else 'null' END
FROM penalties

第二題:統計出屬於low的罰款編號

select * from 
( select paymentno, amount,
 case 
when amount >= 0 AND amount < 40 then 'low'
 when amount >=40 AND amount < 80 then 'moderate'
 when amount >=80 then 'high' 
else 'incorrect' end lvl
 from penalties) as p
where p.lvl = 'low'

PS：Mysql，Case When,Case多個字段

select distinct a.PatientID,a.PatientCode,a.PatientSex,a.MobileNo,a.HomePhoneNo,a.UserAge,a.PatientName,a.PatientIDCard, DATE_FORMAT(a.RegistDate,'%Y-%m-%d') as RegistDate, 
 case when b.usedstartTime is not null and b.UsedEndTime is null then '1'
when b.usedstartTime is not null and b.UsedEndTime is not null then '2' 
 end as 'usedState'
 from mets_v_patient_baseinfo a 
 left join mets_devices_used_history b on a.patientid = b.PatientID
 where  (select ifnull(IsDeleted,0) from userpublic_info where UserID = a.PatientID ) = 0 
 and 1=1 
 order by PatientID Desc limit 0,15